This invention relates to vehicle steering apparatus and, more particularly, to vehicle steering apparatus for steering a vehicle along a predetermined travel path.
The vehicles with which this invention may be used typically are employed in an automated transportation system in which individual vehicles travel along a guideway between predetermined destinations under external, generally computer control. Vehicle steering and, when appropriate, switching of the vehicle at intersections between multiple predetermined travel paths, is accomplished by steering apparatus which is responsive, in alternate sequence, to the contours of two parallel guide surfaces which are formed by the guideway and between which the vehicle travels. (These guide surfaces are referred to herein as the "left guide surface" and the "right guide surface"). The contours of left and right guide surfaces are respectively followed by left and right guide wheels which are mounted on the vehicle adjacent to the sides thereof. A sterring control mechanism is responsive to the positions of the guide wheels with respect to the vehicle and operates the steering mechanism to effect vehicle steering by moving one or more steerable wheels of the vehicle in unison. The steering control mechanism additionally applies an appropriate bias to the left and right guide wheels in alternate sequence, causing either the left or right guide wheel, as the case may be, to contact and maintain contact with the left guide surface or the right guide surface, respectively, as the vehicle moves along the guideway to obtain desired directional guidance of the vehicle. An exemplary transportation system and vehicle of this type is disclosed in U.S. Pat. No. 3,643,601. As used herein the terms "left follower mode" and "right follower mode" refer to operation of the steering apparatus wherein the left guide wheel and the right guide wheel are in contact with and follow the contour of the left and right guide surfaces, respectively.
Until this invention, steering apparatus for vehicles of the type described customarily utilized hydraulic actuation and control of the guide wheels and vehicle steerable wheels to obtain directional guidance with respect to the guideway. Non-hydraulically operated steering controls and especially spring operated steering controls were considered unsatisfactory because of fears that they required unacceptably large magnitude control or operational forces in order to overcome frictional energy losses, or lacked sufficient accuracy, reliability or sensitivity for automatic control purposes. One such steering apparatus includes a spring-biased, over-center linkage and a bi-directional reciprocative actuator which selectively positions the over-center linkage to apply a left bias force or a right biasing force to the left or right guide wheels, as the case may be. The biasing forces are provided by two sources--a movable spring and a bi-directional hydraulic power steering cylinder. The spring exerts an over-center bias on the cover-center linkage which positions the spring responsive to operation of the positioning actuator so that it will apply bi-directional rotational effort to a rotatable control arm connected between the guide wheels and the steerable wheel(s). The rotational effect obtained appears as biasing forces at the guide wheels. As a consequence, the over-center bias exerted by the spring is transmittable to the guide wheels in alternate sequence as a left bias force and a right bias force, depending upon the over-center position of the spring.
The resultant bias force obtained at each guide wheel, however, is controlled in relation to desired side load by counteractive operation of the second source of spring force--the power steering cylinder--with respect to the spring in accordance with customary practice. The power steering cylinder continuously counters the overcenter spring in phase with the side load applied by the biased guide wheel to the cooperative guide surface to control the side load applied. A control valve acting between the control arm and the guide wheels and responsive to movement of the guide wheels controls hydraulic fluid pressure applied to the power steering cylinder in phase with the side load applied. In an exemplary application, the power steering cylinder continuously counteracts at least 50% of the force generated by the overcenter spring. In the event the powr steering cylinder or control valve become inoperative, however, the side load applied exceeds acceptable limits, with attendant increase in tire wear and decline in steering effectiveness. In some practical cases, moreover, hysteresis and lead-lag factors associated with the control valve may tend to cause the power steering cylinder to become out-of-phase with the side load applied under many operational conditions, especially in the presence of sine sweeps or during translation of the vehicle from a left to a right follower mode. In some instances, shock loads or even relatively minor impacts to the follower wheels cause the control valve to throw the power steering cylinder so far out-of-phase that violent oscillations occur. Other simplified hydraulic control systems suitable for economic mass production and reliable low maintenance operation are likewise unsatisfactory for providing necessary hysteresis and phasing to overcome these deficiencies. More complex hydraulic or combination electro-hydraulic control systems, of course, could be used, although these systems increase the complexity and cost of the steering apparatus to the point that it is uneconomical and lacks desired reliability.